Since this is in biology instead of philosophy, what predictions does your theory make? If you hold up two fingers in front of a group of people and ask them how many fingers you're holding up, what answers would you expect to get?

Well, in the case of certain mental illnesses, it would be true that perception of reality is altered. Hallucinogens would also have such an effect.

As for healthy, non-drug treated people...well, we really don't know, do we? Though, the example you're giving is more one of what are we paying attention to and noting, not whether two people looking at the same thing are perceiving it differently.